The present invention relates generally to a joint structure for joining electrical cables, and more particularly, concerns the insulation structure of a heat shrinkable joint for electrical cables and a method for its manufacture, the joint being particularly useful for high and very high voltage applications.
Numerous methods and joint structures for joining electrical cables are known. However, heat shrinkable, polymeric insulated joints are especially useful for joining extruded electrical cables, because the joints do not have to be manufactured to the same diameter as the cables to be joined. Heat shrinkable joints are also advantageous because they can be applied in a relatively short time.
Existing heat shrinkable joint structures usually include a number of coaxial layers of heat shrinkable tubing extending between the cables being joined. Typically, there is an inner heat shrinkable tube over which one or more layers of additional heat shrinkable tubes are provided, the innermost part of the composite structure having semiconducting characteristics, followed by an insulating layer and another semiconducting layer. Such joints exhibit relatively low dielectric strength and questionable reliability. Heat shrinking a tube over a substrate or over another heat shrinkable tube may lead to the formation of gas bubbles or blisters in the interface between the layers. It may also trap contaminants picked up during different steps in the process. In high voltage applications, partial discharge may occur in these gas bubbles, or voltage enhancement by certain cotaminants may occur which will lead to premature failure of the joint.
Broadly, it is an object of the present invention to overcome the shortcomings of prior art joints and especially of shrinkable joint structures for electrical cables. It is specifically contemplated that such a joint structure be provided which has extremely high electrical strength as high as that of the cables it joins, short length, and efficient longitudinal voltage distribution.
It is another object of the invention to avoid the formation of gas bubbles under the heat shrinkable layers in a heat shrinkable joint structure for cables, thereby avoiding partial discharge in high voltage applications and the associated premature joint failure.
It is also an object of the invention to provide a heat shrinkable joint structure for electrical cable and a method for making the same which are reliable and convenient in use yet relatively inexpensive.
In accordance with an illustrative embodiment demonstrating objects and features of the invention, a multilayer, heat shrinkable composite tube covering is provided for a cable joint structure in which intimately joined layers define a semiconductive inner or conductor shield, an insulation layer, and an outer or insulation shield for the joint. Furthermore, a highly resistive semiconducting layer is provided over the joint under the innermost layer of the composite tube, forming a dual layer conductor shield for the joint.
In the preferred embodiment, the entire composite tube is pressure extruded in a common operation and all three layers are simultaneously chemically cured and cooled under pressure to provide the intimately joined layers of the composite tube. Subsequently, when the tube is shrunk over the joint, the three layers shrink as a unit, so that there is no possibility of trapping impurities or gas bubbles between layers.